As the largest organ of the human body, skin provides a painless and compliant interface for systemic drug delivery (Prausnitz et al., 2004, Nature Rev. Drug Discov. 3:115; Thomas and Finnin, 2004, Drug Discov. Today 9:697; and Zaffaroni, 1991, Ann. N.Y. Acad. Sci. 618:405). However, the permeability of foreign molecules, especially large hydrophilic molecules, across the skin is extremely low, primarily due to the presence of the stratum corneum, a unique hierarchical structure of lipid-rich matrix with embedded corneocytes at the outer surface of skin (Scheuplein and Blank, 1971, Physiol. Rev. 51:702).
Various chemical penetration enhancers have been studied in an attempt to open up the skin barrier but with limited success (Williams and Barry, 2004, Adv. Drug Deliv. Rev. 56:603; Purdon et al., 2004, Crit. Rev. Ther. Drug Carrier Syst. 21:97; Kanikkannan et al., 2000, Curr. Med. Chem. 7:593; and Finnin and Morgan, 1999, J. Pharm. Sci. 88:955). Without the aid of physical enhancement means such as iontophoresis (Kalia et al., 2004, Adv. Drug Deliv. Rev. 56:619; Hirvonen et al., 1996, Nat. Biotechnol. 14:1710), ultrasound (Lavon and Kost, 2004, Drug Discov. Today 9:670; Mitragotri et al., 1995, Science 269:850) or microneedles (McAllister et al., 2003, Proc. Natl. Acad. Sci. U.S.A. 100:13755; Prausnitz, 2004, Adv. Drug Deliv. Rev. 56:581), chemical penetration enhancers are generally unable to deliver therapeutic levels of large (>500 Da) hydrophilic drugs through intact skin to the systemic circulation (Bos and Meinardi, 2000, Exp. Dermatol. 9:165). Finding novel skin penetration enhancers that can overcome this limitation would significantly advance the current state of transdermal drug delivery.
In vivo phage display has been used to identify organ- and tissue-targeting peptides (Pasqualini and Ruoslahti, 1996, Nature 380:364; Arap et al., 1998, Science 279:377; Raffii et al., 2003, Cancer Cell 4:331; Kolonin et al., 2004, Nature Med. 10:625). In general, phage display describes a selection technique in which a library of variants of a peptide or protein is expressed on the outside of a phage virion, while the genetic material encoding each variant resides on the inside (Sidhu et al., 2003, Chembiochem. 4:14; Ferrer et al., 1999, J. Pept. Res.: 54, 32; BouHamdan et al., 1998, J. Biol. Chem. 273: 8009). This creates a physical linkage between each variant protein sequence and the DNA encoding it, which allows rapid partitioning based on binding affinity to a given target molecule (antibodies, enzymes, cell-surface receptors, etc.) by an in vitro selection process called panning (Whaley et al., 2000, Nature, 405, 665). In its simplest form, panning is carried out by incubating a library of phage-displayed peptides with a plate (or bead) coated with the target, washing away the unbound phage, and eluting the specifically bound phage. The eluted phage is then amplified and taken through additional binding/amplification cycles to enrich the pool in favor of binding sequences. After 3-4 rounds, individual clones are characterized by DNA sequencing and ELISA. However, the in vivo phage display method has not been used to identify peptides with transdermal capability.
Various drug molecules (Grama and Bouwstra, 2002, J. Controlled Release 83:253; Grams et al., 2004, J. Controlled Release 98:367), microspheres (Rolland et al., 1993, Pharm. Res. 10:1783) and liposome formulations (Li and Hoffman, 1995, Nature Med. 1:795; Hoffman, 1997, J. Drug Targeting 5:67) were known to exhibit follicular penetration, and hair follicles are increasing being recognized as an important route of entry for transdermal drug delivery (Lauer et al., 1995, Pharm. Res. 12:179; Agarwal et al., 2000, Methods Find. Exp. Clin. Pharmacol. 22:129). However, the definite proof for transfollicular delivery has been difficult to obtain (Meidan et al., 1998, Pharm. Res. 15:85).
A class of membrane-permeable peptides, called Protein Transduction Domains (PTDs; Joliot and Prochiantz, 2004, Nature Cell Biol. 16:189), have been reported to facilitate epicutaneous delivery of protein and peptide molecules (Rothbard et al., 2000, Nature Med. 6:1253; Schutze-Redelmeier et al., 2004, Vaccine 22:1985; Lopes et al., 2005, Pharm. Res. 22:750; Lim et al., 2003, J. Cosmet. Sci. 54:483). PTDs, however, deliver cargo only locally and not systemically, and they require physical association (usually achieved through covalent linkage) with the cargo to fulfill the delivery function.
There is an ongoing need, therefore, for developing transdermal enhancers that are highly effective in enhancing and/or facilitating a drug to permeate the skin so that the amount of the drug in the systemic circulation or reached to the target organs, tissues, or cells is increased. In addition, using the transdermal enhancers to enhance and/or facilitate the transdermal delivery of drugs does not result in skin damage, irritation, sensitization, systemic toxicity, or the like, and can be used to effect transdermal delivery of even high molecular weight drugs such as peptides, proteins, and nucleic acids.